The present invention relates to polyester polymerization. More specifically, the invention relates to a process for preparing flame-retardant polyesters from halogenated polymerization reactants.
Polyesters can be flame-retarded by either of two basic approaches. One approach has been to distribute throughout the polymer a flame-retardant which does not enter into a polymerization reaction with the polyester intermediates or the polymerized polymer. A second approach has been to incorporate into the polyester polymer molecule the flame-retardant as an integral constituent of the molecule. As an example, phthalic anhydride is a common polyester acid intermediate which, when copolymerized with a polyol such as ethylene glycol, forms a polyester. By substituting a halogenated polybasic acid anhydride for the unsubstituted phthalic anhydride, the flame-retarding halogen atoms can be incorporated into the polymer molecule and hence become less subject to exudation and migration than are many flame-retardants which are only physically incorporated into the polymerized product. Tetrabromophthalic anhydride is an example of one halogenated aromatic polybasic acid intermediate which has been frequently used in the production of flame-retardant polyesters. Tetrachlorophthalic anhydride is another example. 1,4,5,6,7,7-hexachlorobicyclo[2,2,1]-5-heptene-2,3-dicarboxylic acid, commonly referred to as HET acid, is a halogenated aliphatic acid intermediate which is also used in flame-retardant polyester synthesis. In addition to the use of either an aromatic or an aliphatic acid intermediate in polyester synthesis, a mixture of two or more halogenated aromatic and aliphatic intermediates is often used advantageously to produce flame-retardant polyesters having some desirable combination of cost and flame-retardancy which cannot be attained through the use of either of the halogenated intermediates alone.
When halogenated polybasic acids or anhydrides are used in the synthesis of polyesters, it has often been necessary to neutralize inorganic acid residues, such as sulfuric acid or sulfur trioxide residues in the polybasic acids, before the polymerization reaction with the polyol is permitted to proceed. Tetrahalophthalic acids and anhydrides sometimes contain inorganic acidic residues which should be removed or inactivated before the acids or anhydrides are polymerized with polyol intermediates. The neutralization has customarily been conducted with a base sufficiently strong to neutralize the residual inorganic acid. Alkali metal salts of alkanoic acids such as sodium acetate have been effective in such applications as have certain organic bases such as amines and the like. When one or more halogenated aromatic polybasic acids or anhydrides are to be neutralized prior to their polymerization, a large number of bases have customarily been used to neutralize undesirable acid residues without materially affecting the halogenated aromatic polybasic acid's or anhydride's ability to enter into a subsequent polyester polymerization. One significant measure of the effect of the neutralizing base upon the acid or anhydride, in addition to the type of polymerized product formed, is a measurement of the color of the neutralized polymerization system after polymerization. A light color, similar to the color of the polymerization reactants before neutralization, is indicative of a neutralization which has caused little decompositional change in the polymerization reactants. A dark color, however, indicates that the neutralizing agent has reacted in such a way with the acid or diol intermediate as to produce decomposition products which may affect such properties of the polyester resin as impact strength, tensile strength and the like. Apart from any effect of the decomposition products upon strength properties, the dark color of the polymerization system, which is imparted to the polymeric product, is itself an undesirable property.
Many of the customary neutralizing agents used very effectively with halogenated aromatic acids alone are unsatisfactory when used with a mixture of halogenated aromatic and aliphatic acids because of the dark color produced in the polymerization system. Since, as pointed out above, mixtures of halogenated aromatic and aliphatic acids are desirable for certain applications, a means for neutralizing such mixtures without producing a discolored polymerization system and the consequent discolored polymerization product would constitute a valuable addition to the art. Providing such a means is one of the principal objects of this invention. Additional objects will become apparent from the detailed description of the invention which follows.